Resilient, metallic sealing rings have the ability to repeatedly accommodate large variations in length due to thermal expansions and contractions of assemblies by which they are surrounded and with which they are engaged in sealing contact. In the hot sections of an aircraft gas turbine engine, a seal must be created between components that are at variable distances from one another, depending upon the sum of manufacturing tolerances on several components in the assembly and on relative thermal expansion during operation of the engine.
Convolution-type sealing rings are particularly well suited for application described above. Examples include U.S. Pat. Nos. 3,797,836 and 4,121,843 to Halling. Sealing rings described therein have a single-ply construction. Their use is generally limited by the amount of axial deflection to which they can be subjected without being plastically deformed, and by the amount they are caused to relax at high temperatures or prone to metal fatigue. They must also be manufactured in material of sufficient thickness to avoid excessive stresses due to pressure or deterioration due to oxidation.
Thin wall single-ply seals are sometimes necessary to maintain the stress due to deflection at levels low enough to avoid failure of the seal. This is the case where the amount of deflection, due to installation tolerances and cyclic thermal excursions of the cavity walls, is very large. These thin wall seals tend to be weak and exhibit a low resonant frequency, which can result in vibration induced high cycle fatigue and/or fretting wear failures.
Multi-ply seals are generally known, but these have independently acting plies, in which the plies are not joined together by welding, brazing, etc. One example is found in U.S. Pat. No. 3,012,302 to Waite, wherein a sealing ring has three separate elements nested together to form a three-ply structure. The opposite free ends of the three plies are formed to extend axially so that the sealing line passes through the free ends. Since the free ends are not connected to each other, high pressure fluid may enter the interstitial spaces between the plies. This could result in a higher pressure-induced stress in the outer ply. The stress would be doubled, for example, in the case of a two-ply sealing ring.
Examples of multi-ply annular seals are disclosed in U.S. Pat. No. 2,263,756 to Bowers, U.S. Pat. No. 3,595,588 to Rode, U.S. Pat. No. 4,361,335 to Vinciguerra, U.S. Pat. No. 4,477,086 to Feder, and U.S. Pat. No. 4,218,067 to Halling. These patents do not show annular plies connected to each other at their free ends.